The present invention relates to a speed control apparatus in a magnetic disk drive apparatus for generating a speed signal from sinusoid-like position signals, thereby controlling the position of a magnetic head in the magnetic disk apparatus.
In a conventional magnetic disk system, a magnetic head is moved to a track position designated by a seek instruction, based on servo data pre-recorded on a magnetic disk. In accordance with this positioning scheme, when the magnetic head is to be moved to the designated track position, servo data pre-recorded on the servo surface of the magnetic disk is read by the magnetic head to generate the position signal and designated speed data. A generated speed signal is differentiated to detect a designated moving speed signal of the magnetic head. The detected speed signal is compared with the designated speed data. In this manner, the magnetic head can be driven at a proper speed.
For example, the technique for generating the position signal based on the servo data pre-recorded on the magnetic disk is disclosed in U.S. Pat. No. 4,238,809, and the technique for generating the speed signal by differentiating the position signal is disclosed in U.S. Pat. No. 3,839,665.
The servo data-based position signal corresponding to each data track on the magnetic disk surface is an alternating signal of an interval of several tracks, for example, a sinusoid-like signal as a function of the track position. If the phase of the sinusoid-like position signal has to be changed during driving of the magnetic head, the differentiation operation for deriving the speed signal causes generation of a surge component in the speed signal. The surge component is independent of the actual speed, and if the speed signal including the surge component is used without being corrected, the magnetic head can not operate properly during speed control.